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Chapter 3 Atoms and Elements. 3.4 Atomic Number and Mass Number. Atomic Number. The atomic number is specific for each element is the same for all atoms of an element is equal to the number of protons in an atom appears above the symbol of an element. 11 Na. Atomic Number. Symbol.

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chapter 3 atoms and elements
Chapter 3 Atoms and Elements

3.4Atomic Number and Mass Number

atomic number
Atomic Number

The atomic number

  • is specific for each element
  • is the same for all atoms of an element
  • is equal to the number of protons in an atom
  • appears above the symbol of an element

11

Na

Atomic Number

Symbol

atomic number and protons
Atomic Number and Protons

Examples of atomic number and number of protons:

  • Hydrogen has atomic number 1; every H atom has one proton.
  • Carbon has atomic number 6; every C atom has six protons.
  • Copper has atomic number 29; every Cu atom has 29 protons.
  • Gold has atomic number 79; every Au atom has 79 protons.
learning check
Learning Check

State the number of protons in each.

A. A nitrogen atom

1) 5 protons 2) 7 protons 3) 14 protons

B. A sulfur atom

1) 32 protons 2) 16 protons 3) 6 protons

C. A barium atom

1) 137 protons 2) 81 protons 3) 56 protons

solution
Solution

State the number of protons in each.

A. A nitrogen atom

2) atomic number 7; 7 protons

B. A sulfur atom

2) atomic number 16; 16 protons

C. A barium atom,

3) atomic number 56; 56 protons

number of electrons in an atom
Number of Electrons in an Atom

An atom

  • of an element is electrically neutral; the net charge of an atom is zero
  • has an equal number of protons and electrons.

number of protons = number of electrons

  • of aluminum has 13 protons and 13 electrons; the net charge is zero

13 protons (13+) + 13 electrons (13–) = 0

mass number
Mass Number

The mass number

  • represents the number of particles in the nucleus.
  • is equal to the

number of protons + number of neutrons

learning check1
Learning Check

An atom of zinc has a mass number of 65.

A. How many protons are in this zinc atom?

1) 30 2) 35 3) 65

B. How many neutrons are in the zinc atom?

1) 30 2) 35 3) 65

C. What is the mass number of a zinc atom that has

37 neutrons?

1) 37 2) 65 3) 67

solution1
Solution

An atom of zinc has a mass number of 65.

A. How many protons are in this zinc atom?

1) 30 (atomic number 30)

B. How many neutrons are in the zinc atom?

2) 35 (65 – 30 = 35)

C. What is the mass number of a zinc atom that has 37 neutrons?

3) 67 (30 + 37 = 67)

learning check2
Learning Check

An atom has 14 protons and 20 neutrons.

A. Its atomic number is

1) 14 2) 20 3) 34

B. Its atomic number is

1) 14 2) 20 3) 34

C. The element is

1) Si 2) Ca 3) Se

solution2
Solution

An atom has 14 protons and 20 neutrons.

A. Its atomic number is

1) 14

B. Its atomic number is

3) 34 (14 + 20 = 34)

C. The element is

1) Si (atomic number 14)